Lubricant Additives

100 Lubricant Additives: Chemistry and Applications

This behavior as secondary antioxidants by destroying the hydroperoxides, etc., formed in the
chain propagation process results in their use in synergistic combination with those antioxidant
types that are active as radical scavengers in the initiation process, for example, the hindered phe-
nols and aromatic amines [134–138].
Phosphites are useful additives because of their multifunctionality. However, although they
are still used as antioxidants in hydrocarbon oils, their relatively poor hydrolytic stability and
the formation of acidic compounds that could affect the surface active properties of the oil have
prompted the introduction of “hindered” phosphites with better hydrolytic stability: for example,
tris-(2,4-ditertiarybutylphenyl) phosphite or tris-(3-hydroxy-4,6-ditertiarybutylphenyl) phosphite,
and, where solubility permits, cyclic phosphites, for example, based on pentaerythritol such as bis-
(2,4-ditertiarybutylphenyl) pentaerythritol diphosphite (Figure 3.18). These types are claimed as
stabilizers or costabilizers for lubricating oils [139–142].
Table 3.14 [141] illustrates the signifi cant improvement in oxidation stability shown by such
blends.
In common with most other types of phosphorus-containing products, neutral (and acid) phos-
phites have also been claimed as corrosion inhibitors [143,144].

3.5.5 ALKYL AND ARYL ACID PHOSPHITES

As might be predicted from the behavior of the other types of phosphorus-containing additives, the
acid phosphites have good AW/EP properties; the nonylphenyl acid phosphite is particularly effec-
tive [145,146]. When used in aviation gas turbine lubricants, the acid phosphites were sometimes
formulated in combination with neutral phosphates (TCP); blends of the two products showed syn-
ergy even when the amount of the phosphite was very low [147]. The acid phosphites are also
claimed to be corrosion inhibitors [144] and antioxidants [47,149,150].
The effects of structure on the AW and load-carrying properties of dialkyl phosphites was
studied by Forbes and Battersby [151] in a liquid paraffi n. AW performance was best with long-
chain compounds (Figure 3.19), whereas the short-chain (highest phosphorus content) derivatives
displayed the best load-carrying performance.
Scuffi ng behavior, however, appeared to reach a minimum at about a C 8 carbon chain length
(Figure 3.20). This parallels the behavior of the neutral phosphites. Adsorption studies also showed
that the phosphorus content of the solution was depleted in the same order of the load-carrying
performance, namely, the most active products showed the highest loss from solution. The pres-
ence of water increased the uptake of phosphorus from solution. Comparison of the performance

P

PP

O

O

O

O

O

O

O

O

Tris-(2,4-ditertbutylphenyl) phosphite

Bis-(2,4-ditertbutylphenyl) pentaerythritol diphosphite

O

FIGURE 3.18 Structures of some commonly available hindered phosphites.